Study of the amorphization process of MgSiO3 by ion irradiation as a form of dust processing in astrophysical environments

The study of the crystalline-to-amorphous transition of silicates triggered by ion irradiation is useful for the understanding of the structural modification of interstellar dust grains. We focused our experiments on the mineral enstatite (MgSiO3), an essential part of the polycrystalline silicate dust. Within our studies He+-, Li+-, Ne+- and Ar+-ions with energies between 50 and 400 keV were used. Irradiations were carried out at 70 K. The samples were analyzed by transmission electron microscopy in combination with electron diffraction and infrared reflection spectroscopy. It was found, that the number of displacements necessary to amorphize enstatite is higher for lower ion masses. The amorphization threshold increases from 0.3 to about 5 dpa in the case of 400 keV Ar+- and 50 keV He+-ions, respectively. This can be explained by the dilution of the collision cascades with decreasing ion mass and with an in situ annealing process with enhanced ratio between electronic and nuclear energy loss, respectively